Re: phantom memory in a cgroup (was [BUG] ZSwap leaks memory upon being disabled)

[Yosry Ahmed <yosryahmed@google.com>]

On Thu, Oct 31, 2024 at 2:59 PM Konstantin Kharlamov <Hi-Angel@yandex.ru> wrote:
>
> On Wed, 2024-10-30 at 12:44 -0700, Yosry Ahmed wrote:
> > shmem increased by a lot (~1.8G).
> >
> > So this looks like it could be the answer to your question about
> > where
> > the swap usage is coming from. I would try to find what tmpfs files
> > are used by this application.
>
> Thank you! After doing more digging I reduced it to `Xorg` having a
> hunderds of `anon_inode:i915.gem`, and afterwards pinned down this to
> be Picom not freeing resources. Reported on Github¹.
>
> That said, isn't there a kernel bug too? If this `shmem` ends up in
> Swap, then it should be accounted in `Swap` fields of
> `proc/<pid>/smaps` accordingly, right? In the end, that's what the
> field is for: amount of SWAP taken by a process. Otherwise it is a
> "phantom memory": something being in SWAP, but who owns this
> "something" — there's no way to know, it just kind of "exists" amidst
> kernel and processes realms.

I don't think so. shmem doesn't really belong to a single process. If
you kill the process but leave the tmpfs files behind, the memory will
not go away.

>
> 1: https://github.com/yshui/picom/issues/1378